Post-treatment Larval Cultures Research Articles

Resistance of Gastrointestinal Nematodes to Anthelmintics in Sheep Production in Zimbabwe

Introduction: Sheep play a crucial role in enhancing the socio-economic welfare of individuals in numerous nations. Within sub-Saharan Africa, productivity levels are hindered by various factors, such as prevalent diseases, inadequate nutrition, climate variations, predators, severe weather, as well as infestations of both external and internal parasites. The purpose of the present study was to assess the extent of anthelmintic resistance (AR) observed on a sheep farm operating for commercial purposes in Zimbabwe. Materials and methods: Forty female weaner Dorper sheep, averaging 43 kg in body weight, were selected from Ballineety commercial sheep farm in Mashonaland Central, Zimbabwe. These sheep were then divided into four groups of ten each, receiving different treatments including ivermectin, albendazole, levamisole, and no treatment as a control. Each treatment was administered according to the manufacturer’s recommendations. Pre-treatment faecal samples (day zero) and post-treatment (day 14) were collected and subjected to the Faecal Egg Count Reduction Test (FECRT). Anthelminthic resistance was suggested (AR) by the presence of faecal eggs after treatment. Results: All the drugs indicated a decrease in egg counts by less than 90%. The effectiveness of ivermectin, albendazole, and levamisole was measured at 58.5%, 70.1%, and 85.8% respectively. These findings indicate that nematodes have developed resistance to all treatments. The larval cultures of the samples before and after treatment indicated a significant presence of Haemonchus spp. in the pooled samples. In contrast, the quantities of Trichostrongylus spp. and Oesophagostomum spp. were low but present in all treatments. Cooperia spp. was recovered in post-treatment larval cultures of groups that received anthelmintic. The present study reports lower-than-expected efficacy for ivermectin, albendazole, and levamisole based on the reduction of egg excreted post-treatment. Conclusion: A common worms population in sheep are AR in the current study area. Therefore, comprehensive nematode management programs along with complementary eradication strategies are essential for sustainable sheep production.

MULTIDRUG RESISTANCE OF SHEEP GASTROINTESTINAL NEMATODES IN BAKRAJO DISTRICT, NORTH IRAQ TO ALBENDAZOLE, IVERMECTIN, AND LEVAMISOLE

A survey including 25 sheep farms in Bakrajo District, Sulaymaniyah, Iraq, was conducted to determine anthelmintic resistance (AR) against albendazole, ivermectin, and levamisole. Fecal Egg Count Reduction Test (FECRT) was used to accomplish this goal. Forty 6–24 month-old sheep of mixed sexes were selected from each farm and randomly divided into four groups. One group served as the untreated control, and the other groups orally administered the recommended doses of the tested drugs. Multidrug resistance against all three drugs was present in 8 of 25 farms (32%). Also, resistance to albendazole and ivermectin was detected on one farm. Albendazole was the most effective anthelmintic agent. Resistance to ivermectin was significantly higher than levamisole and albendazole. Larval cultures revealed that Trichostrongylus spp., Nematodirus spp., and Trichuris spp. were the prevalent gastrointestinal nematodes in the study area. The nematode genera were also detected in the posttreatment larval cultures. It is concluded that AR to albendazole, ivermectin, and levamisole is widespread in sheep in Bakrajo District, north Iraq. This resistance is mainly caused by the emergence of resistant Trichostrongylus and Nematodirus spp.

Anthelmintic resistance in commercial sheep farms in Costa Rica.

Forty-eight commercial sheep farms were sampled between the years 2017 and 2019 to evaluate the percentage of reduction of the three families of broad-spectrum anthelmintic drugs available in Costa Rica - benzimidazoles (albendazole), macrocyclic lactones (ivermectin) and imidazothiazoles (levamisole) - in grazing animals by using the fecal egg count reduction test. Animals were selected based on their egg per gram counts after their fecal samples were processed by a modified McMaster technique. The remaining fecal samples were taken to the laboratory for fecal culture and larval identification. The prevalence of farms with albendazole resistance was 100%, ivermectin 96% and levamisole 17%. Seven (n=47) farms resulted with triple resistance, 39 with double and 1 farm was resistant to 1 AH tested. Haemonchus spp. was the most commonly isolated parasite in pre- and post-treatment larval culture. This is the first report of Oesophagostomum spp. and Teladorsagia sp. resistant to albendazole, Oesophagostomum spp., Teladorsagia sp., Chabertia sp. and Cooperia sp. resistant to ivermectin, and the presence of strains of Haemonchus spp. and Trichostrongylus sp. resistant to the 3 anthelmintics tested. The results of this study demonstrate that anthelmintic resistance is widespread, particularly in Haemonchus spp., and that the major drugs available in the country have lost much of their effectiveness against sheep nematodes.

Anthelmintic resistance among gastrointestinal nematodes of cattle on dairy calf to beef farms in Ireland

BackgroundThe control of gastrointestinal nematodes (GIN) of cattle in pasture-based production systems such as Ireland is highly dependent on the availability of efficacious anthelmintics. There is very little information available on the efficacy of the broad-spectrum anthelmintics against GIN of cattle in Ireland and the aim of this study was to determine the prevalence of anthelmintic resistance on dairy calf to beef farms.ResultsGIN burden was monitored on thirty-six recruited farms by performing herd level faecal egg counts (FEC) every 2 weeks. Of these, nine farms were lost from the study as calves were treated with an anthelmintic for Dictyocaulus viviparus, two were lost as they treated for GIN, one dropped out of the study and on one the herd FEC did not reach the threshold for carrying out the Faecal Egg Count Reduction Test (FECRT). On the remaining 23 farms, once the herd FEC reached 100 eggs per gram, a FECRT was carried out. Pre and post-treatment larval cultures were also performed to identify the GIN to genus level. The efficacy of fenbendazole, levamisole, ivermectin and moxidectin was evaluated on 15, 11, 16 and 11 farms respectively. Resistance to fenbendazole was identified on 9 farms (60%) with resistance suspected on a further farm. Resistance to levamisole, ivermectin and moxidectin was detected on 2 (18%), 16 (100%) and 8 (73%) farms respectively. The predominant genera detected pre and post-treatment were Cooperia and Ostertagia with both genera detected post-treatment with fenbendazole and ivermectin. Due to the low proportion of Ostertagia spp. pre-treatment, the efficacy of levamisole or moxidectin against this genus could not be reliably established.ConclusionsAnthelmintic resistance was widespread on the sampled dairy calf to beef farms in Ireland with resistance to benzimidazole, levamisole, ivermectin and moxidectin detected.

Anthelmintic resistance in gastrointestinal nematodes of goats in southern Mozambique

Small ruminant production is significantly constrained by gastrointestinal parasites once they cause serious production and economic losses for both small-scale and large-scale farmers in the developing world. The control of helminth parasites is most exclusively based on the use of anthelmintics for the economic production of ruminants. However, the resistance of nematodes to the commonly used groups of anthelmintics represents a threat to the production, particularly for small ruminants. The objective of this study was to assess the efficacy of albendazole, the most frequently used anthelmintic in Gaza and Maputo provinces in the southern region of Mozambique, in gastrointestinal nematodes of goats between November and December, 2016. Eleven goat farms in Gaza (n = 5) and Maputo (n = 6) were surveyed. The faecal egg count reduction test was used to assess the efficacy of the drug. The flocks were considered resistant when the reduction in eggs per gram of faeces was less than 95% and the lower limit of the confidence interval was less than 90%. Resistance to albendazole was detected in 60% (3/5) of the farms in Gaza province and 83.3% (5/6) of the farms in Maputo province. The percentage of faecal egg count reduction varied from 51 to 97% in Maputo and from 0 to 100% in Gaza in the farms surveyed. In pre-treatment coprocultures, Haemonchus spp., Oesophagostomum spp. and Trichostrongylus spp. were the predominant nematode species. Post-treatment larval cultures indicated that Haemonchus spp. and, to a lesser extent, Oesophagostomum spp. and Trichostrongylus spp., were resistant to albendazole. This study provided further evidence that anthelmintic resistance of gastrointestinal nematode parasites in goats is currently a problem of great significance in this region of the country and that appropriate measures must be taken to reverse the situation. Key words: Albendazole, efficacy, gastrointestinal parasites, small ruminants, Mozambique.

Confirmation of ivermectin resistance in Ostertagia ostertagi in cattle in New Zealand

Six suspected cases of ivermectin resistance in Ostertagia spp. in cattle were investigated after routine anthelmintic efficacy testing on commercial farms. On four farms a comprehensive faecal egg count reduction test (FECRT) was undertaken using oral formulations of ivermectin (0.2mg/kg), albendazole (10mg/kg) and levamisole (7.5mg/kg) while on two farms only ivermectin was tested. The proportions of Ostertagia spp. in the untreated control and post-treatment larval cultures were used to apportion egg counts to genera and determine efficacy against this genus.Isolates of Ostertagia spp. recovered from three of the farms were each used to infect 18 six month old calves. The efficacy of oral formulations of ivermectin and moxidectin, both at 0.2mg/kg, was determined against each isolate by slaughter and worm count.The efficacy of ivermectin against Ostertagia spp., based on differentiated FECRT for each of the farms varied from 0% to 88%. The efficacy of ivermectin based on worm counts in the slaughter trial varied from 13% to 75% but moxidectin was >99% effective against all isolates. In addition, in the FECRT albendazole, at a dose rate of 10mg/kg, failed to achieve 95% efficacy against Ostertagia spp. on two farms (82% and 85%). Levamisole consistently failed to achieve 95% efficacy against Ostertagia spp. which is consistent with its known lesser efficacy against this parasite.These results confirm the presence of macrocyclic lactone resistant O. ostertagi in cattle in New Zealand and the likely presence of dual resistance, to macrocyclic lactones and albendazole, in some isolates. Resistant populations of this highly pathogenic parasite are probably not uncommon in New Zealand and pose a significant threat to animal production and welfare in the future.

Benzimidazole resistance in equine cyathostomins in India

Benzimidazole resistance is a major hindrance to the control of equine cyathostominosis throughout the world. There is a paucity of knowledge on the level of benzimidazole resistance in small strongyles of horses in India. In the present study, allele-specific PCR (AS-PCR) that detects F200Y mutation of the isotype 1 β-tubulin gene and faecal egg count reduction test (FECRT) were used for detecting benzimidazole resistance in equine cyathostomin populations in different agro-climatic zones of Uttar Pradesh, India. Results of the FECRT revealed prevalence of benzimidazole resistance in cyathostomins in an intensively managed equine farm in the mid-western plain (FECR=27.5%, LCI=0) and in working horses (extensively managed) at three locations in central plains of Uttar Pradesh (FECR=75.7–83.6%, LCI=29–57%). Post-treatment larval cultures revealed the presence of exclusively cyathostomin larvae. Genotyping of cyathostomin larvae by AS-PCR revealed that the frequency of homozygous resistant (rr) individuals and the resistant allele frequency was significantly higher (p<0.001) in the intensively managed farm in the mid-western plain and in working horses at two locations in central plains of the state. The resistant allele (r) frequency in cyathostomins was significantly higher (p<0.05) in Vindhyan and Tarai and Bhabar zones of Uttar Pradesh. The prevalence of benzimidazole resistant allele (r) was significantly higher (p<0.05) in cyathostomins of intensively managed horses (allelic frequency—0.35) as compared to extensively managed horses (allelic frequency—0.22). The widespread prevalence of benzimidazole resistant alleles in equine cyathostomins in Uttar Pradesh, India, necessitates immediate replacement of the drugs of benzimidazole group with other unrelated effective anthelmintics for management and control of equine cyathostomins.

Prevalence of drug-resistant gastrointestinal nematodes in an organized sheep farm

Aim: The present study was aimed to determine the resistance against albendazole, fenbendazole, levamisole and closantel in gastrointestinal (GI) nematodes of sheep. Introduction: Anthelmintics are used traditionally as an integral part of helminthic control strategies for grazing livestock to prevent production losses from parasitic infections. The continuous and indiscriminate use of the same anthelmintics over years together as the sole means of control are now failing due to the emergence of resistance strains of helminths. Resistance to the commonly used anthelmintics in GI nematodes of sheep has become an increasingly widespread problem throughout the world. Materials and Methods: Fifty-five naturally infected Madras Red lambs of 6-12 months of age were selected and distributed randomly into five treatment groups of 11 animals each. Four groups were treated orally with albendazole (5 mg/kg), fenbendazole (7 mg/kg), levamisole (7.5 mg/kg) and closantel (10 mg/kg) respectively, whereas the fifth group served as untreated control. Fecal samples were collected per rectum of each lamb just prior to treatment (pre-treatment) and on 7, 14, 21 and 28 days post-treatment. The anthelmintic resistance was evaluated by in vivo fecal egg count reduction test (FECRT), post-treatment larval culture and in vitro egg hatch assay. Results: In the FECRT, albendazole reduced the faecal egg count by 86.50%, 84.81%, 85.28% and 84.47% respectively for 4 weeks after treatment. Fecal egg count reduction using fenbendazole was 92.64, 93.04, 90.80 and 90.06% respectively for 4 weeks after treatment. The percent efficacy for levamisole and closantel was more than 95%. The post-treatment larval culture contained only Haemonchus contortus. In the in vitro egg hatch assay, the ED50 value for benzimidazole was 0.299 μg albendazole/ml and levamisole showed an ED50 value of 0.283 μg/ml. Conclusion: Our study confirmed the resistance of H. contortus to benzimidazole in sheep.

Anthelmintic resistance in Swedish sheep flocks based on a comparison of the results from the faecal egg count reduction test and resistant allele frequencies of the β-tubulin gene

A faecal egg count reduction test (FECRT) survey was conducted during the grazing season 2006 and 2007 to provide an updated indication of the prevalence of anthelmintic resistance in sheep flocks in Sweden. A total of 1330 faecal samples from 90 flocks on 45 farms, with a minimum of 20 ewes each, was collected by local sheep veterinarians. Per treatment group, ≈15 lambs were dewormed either with oral suspensions of ivermectin (Ivomec vet. ®) or albendazole (Valbazen vet. ®). The efficacy on each farm was investigated either in 2006 or 2007 by faecal egg counts collected on the day of treatment and in a new sample from the same animals 7–10 days later. Third-stage larvae (L3) were initially identified morphologically from pooled cultures. These were then used as the source of genomic DNA template for two molecular tests. The first was a PCR-based test for specific identification of Haemonchus contortus, and the second was a Pyrosequencing assay for the analysis of benzimidazole (BZ) resistance targeting the P200 mutation in the parasite's β-tubulin gene. Larval cultures indicated that Teladorsagia and Trichostrongylus were the predominant genera, but Haemonchus was diagnosed in 37% of the flocks. The PCR results revealed an almost 100% agreement with those farms that had previously been shown to have Haemonchus present, even when the % prevalence was low (≈3%). Only two (4%) of the surveyed farms showed evidence of BZ-resistant worm populations, with H. contortus being the species implicated according to post-treatment larval culture results. The Pyrosequencing assay detected BZ resistant allele frequencies of >40% in the Haemonchus-positive farms and 100% resistant alleles in the clinically most resistant farms. These preliminary results suggest that the FECRT is less sensitive than the molecular test at detecting BZ resistance. However, both tests need to be interpreted carefully, bearing in mind the relative proportions of species present and the starting egg and/or larval counts. Parasitological diagnosis of “clinical” resistance was also found against ivermectin in two flocks. However, both the pre-treatment FECs and the reductions in these were low, and only three lambs that had between 100 and 450 EPG after treatment were involved.

Evaluation the effect of albendazole against nematodes in sheep in Mosul, Iraq

Six sheep farms in Mosul city, Iraq randomly selected, were surveyed for gastrointestinal nematodes resistant to Albendazole. On each of 6 sheep farms, 20 lambs were randomly distributed into two equal groups untreated control group, and albendazole (benzimidazole) group (10 mg/kg BW). Faecal egg counts and larval cultures were done at 7, 14, and 21 days after anthelmintic treatment. Resistance was apparent for albendazole on 4 farms out of 6 (66.7%). Post-treatment larval cultures indicated: Strongyloides papillosus , Marshalligia marshalli, Nematodirus spathiger and Haemonchus contortus .

Multiple species of nematodes resistant to ivermectin and a benzimidazole-levamisole combination on a sheep farm in New Zealand

AIM: To confirm the presence of multiple anthelmintic resistance on a sheep farm in New Zealand. METHODS: Three groups of 10 weaned Romney-cross lambs were treated either with an oral dose of ivermectin (0.2 mg/kg), or a benzimidazole/levamisole (BZ/LEV) combination (4.75 albendazole and 7.5 mg/kg levamisole), or were left untreated. Ten days later, animals were necropsied, and adult worms recovered and identified from the abomasa and small intestines. Pre- and post-treatment faecal nematode egg counts (FEC) were recorded, and larval cultures were performed. RESULTS: In a faecal egg count reduction test (FECRT), adjusted to reflect pre- and post-treatment larval culture results, ivermectin resistance was detected in Teladorsagia (Ostertagia), Trichostrongylus and Haemonchus spp, while BZ/LEV combination-resistant Teladorsagia and Trichostrongylus spp were also present. Adult worm counts confirmed these results, and identified the species involved as Teladorsagia circumcincta, Trichostrongylus colubriformis and H. contortus. CONCLUSION: Multiple, multi-generic anthelmintic resistance was confirmed on a sheep property in New Zealand. This included the first confirmed case of ivermectin resistance in T. colubriformis from sheep in New Zealand.

First report of multiple drug resistance in trichostrongyles affecting sheep under field conditions in Italy

Drug resistance in sheep gastrointestinal trichostrongyles is a cosmopolitan major constraint to small ruminant production. Despite reports that anthelmintic drug resistance has become common, there are limited information on the presence of drug resistance in Italy. The purpose of this study was to determine the effect of four anthelmintics to control infection in sheep in central Italy. Fifty sheep with fecal egg counts (FEC) > or =150 eggs per gram were selected on each of three farms (n = 150 total sheep) which were randomly allocated to one of five groups. Groups were treated with febantel, levamisole, ivermectin, or moxidectin while the fifth group acted as the control group. A FEC reduction test (FECRT) was conducted on each animal and the mean FEC of each treatment group was compared to that of the control group within farm. Resistance was declared when percentage reduction (R) <95% and the lower 95% confidence interval was <90%. Levamisole (mean R = 89%) resistance was found on all farms and ivermectin (mean R = 93%) resistance was found on two of the three farms. Posttreatment larval cultures showed the presence of Teladorsagia (Ostertagia) circumcincta and Trichostrongylus spp. larvae. Febantel (mean R = 96%) and moxidectin (mean R = 100%) remained effective. This study suggests that drug resistance in sheep gastrointestinal trichostrongyles is present in central Italy and a potential problem which would justify a broader nationwide geographical investigation.

Oxfendazole Resistance in Gastrointestinal Nematodes of Beetal Goats at Livestock Farms of Punjab (Pakistan)

This study was carried out to screen goat farms for anthelmintic resistance (AR) against oxfendazole (OXF) and to determine contributory factors for its development. For this purpose, Beetal goat farms (n = 18) were randomly selected, with natural mixed gastrointestinal nematodosis infection. In vivo (faecal egg count reduction test) and in vitro (egg hatch assay) tests were used to ascertain the presence of AR while a scorecard was used to determine the role of possible contributory factors for oxfendazole resistance. For in vivo test, the experimental animals were divided into two groups of 10 animals each; one group received OXF treatment, while the other served as control. Pre- and post-treatment coproculture was performed to identify the species and genera of nematodes. Egg hatch assay (EHA) was used to confirm the results of FECRT. Fecal egg count reduction test (FECRT) revealed the development of resistance on six farms and post-treatment larval cultures indicated Haemonchus contortus, Trichostrongylus colubriformis, Cooperia curticei, Teladorsagia circumcincta and Oesophagostomum spp. as dominant species with resistance. Furthermore, EHA confirmed the results of FECRT. Among the presumptive factors for AR, the highest composite score was for rotation of anthelmintics followed by treatment frequency, dose rate and nature of medication. The scorecard for the development of AR, used in this study, may be helpful for the assessment of contributory factors of AR.

Prevalence of benzimidazole resistant nematodes in sheep flocks in Yucatan, Mexico

Thirty-eight sheep flocks, located in three municipalities in the Eastern Yucatan, Mexico, were surveyed for gastrointestinal nematodes resistant to benzimidazole (BZD) anthelmintics (AH). On each flock, 30 sheep were randomly distributed into two groups of 15 animals: albendazole group (5 mg/kg BW) and untreated control group. Animals were refrained from any food (either browsing/grazing or supplement) for a period of 16 h prior to treatment. Faecal egg counts (FEC) and larval cultures were performed 10 days after anthelmintic treatment. Percentage reduction and 95% confidence intervals were determined. Anthelmintic resistance (AR) was declared when the percentage reduction in FEC was <95% and the 95% confidence interval was <90%. AR was suspected when only one of the two criteria was met. The survey indicated that AR occurred in 15.8% ( n=6) (95% confidence interval=±11.6%) and was suspected in 23.7% of the farms ( n=9) (95% confidence interval=±13.3%). Post-treatment larval cultures indicated that Haemonchus was the only resistant genus. The questionnaire survey showed that most farmers (92%) considered their sheep a secondary activity to cattle production. The majority of farmers (97.4%) treat their animals according to visual appreciation of weight. However, most farmers (79%) treat their flocks at very low frequencies (<3 times per year). Drug rotation was performed every 12 months or more by 84.2% of farmers. Anthelmintics used were: macrociclic lactones (47.4%), BZD (39.5%), levamisol (10.5%) and 1 farmer used closantel.

Prevalence of anthelmintic resistant nematodes in sheep and goats in Western France

Twenty-three sheep farms and 15 dairy goat farms, located in western France and randomly selected, were surveyed for gastrointestinal nematodes resistant to anthelmintics. On each of the 23 sheep farms, 60 lambs were randomly distributed into four groups of 15: untreated control group, fenbendazole (benzimidazole) group (5 mg/kg BW), ivermectin group (0.2 mg/kg BW) and levamisole hydrochloride group (6.8 mg/kg BW). On each of the 15 goat farms, 30 adult goats were allocated into two groups of 15: untreated control group and fenbendazole group (10 mg/kg BW). Faecal egg counts and larval cultures were done 10 to 14 days after anthelmintic treatment. Furthermore, egg hatch assays conducted with thiabendazole were performed on pooled faeces of fenbendazole treated goats, results being expressed as Logarithmic Concentration 50 (LC 50). Faecal egg count reductions (FECR) less than 95% are indicative of anthelmintic resistance and LC 50 values above 0.1 μg/ml suggest benzimidazole (BZ) resistance. On this basis, in the sheep survey, anthelmintic resistance occurred on 83% of farms for benzimidazole and on 50% of farms for levamisole. Post-treatment larval cultures indicated Teladorsagia/ Trichostrongylus and Cooperia were the predominant resistant genera. A negative relationship was found between FECR with fenbendazole and the average number of annual BZ treatments given to ewe-lambs ( P<0.01). In contrast, no case of ivermectin resistant nematodes was seen in the 23 sheep farms. In goat survey, virtually all the 15 surveyed farms showed the occurrence of benzimidazole resistant nematodes according to the results of both FECR test and egg hatch assay. In post-treatment coprocultures, Teladorsagia/ Trichostrongylus was the predominant larval type on seven farms, and Haemonchus on two farms, while on the remaining farms there was a well-balanced mixture of all the genera including Oesophagostomum/ Chabertia. Although no relationship could be established between drenching practices and results of in vivo and in vitro tests, the frequent and sometimes exclusive use of BZ compounds associated with a permanent underdosing of these drugs achieved by using sheep dose rates are probably the main causes of the very high frequency of BZ resistance in nematodes on goats. The consequences of anthelmintic resistance on sheep and goat parasitism management are discussed.

A survey of anthelmintic resistance on ten sheep farms in Mashonaland East Province, Zimbabwe.

A survey to detect anthelmintic resistance in nematode parasites of sheep was conducted on 10 randomly-distributed farms in the Chivhu District, Mashonaland East Province, Zimbabwe. Before the survey, a questionnaire was circulated to the farmers concerning nematode parasite control. Results showed that parasite control using anthelmintic treatment was the only method practised and that the benzimidazoles were the most frequently used anthelmintic drugs. The faecal egg count reduction test was used to detect resistance. The anthelmintic groups tested were benzimidazoles, levamisole and ivermectin. Resistance to benzimidazoles was detected on 6 of 10 farms and levamisole resistance on 2 of 3 farms. Ivermectin resistance was not observed on the farms surveyed. Post-treatment larval cultures indicated that Haemonchus contortus survived administration of fenbendazole, albendazole, oxfendazole and levamisole. A Cooperia sp. strain resistant to albendazole was detected and this is the first report in Zimbabwe of a resistant parasite in this genus.

Misconceptions regarding the use of post-treatment larval cultures for the identification of anthelmintic-resistant sheep nematodes

Extract In a recent communication(1), it was suggested that, without a corresponding measure of their initial abundance also, the results of post-treatment larval cultures undertaken to determine the identities of those nematode genera surviving an- thelmintic treatment during faecal egg count reduction (FECR) testing in sheep were likely to be largely meaningless. Since then, the validity of this statement has been queried by a number of veterinary practitioners who appear to assume that, in cases of anthelmintic resistance, the only nematode genera that might be expected to be represented in post-treatment larval cultures are those that are drench-resistant. That this latter assumption is unfounded, however, may be illustrated by an analysis of post-treatment larval culture results obtained from 102 ovine anthelmintic resistance cases identified by FECR testing at the Batchelar Animal Health Laboratory.

Survey for drug-resistant gastrointestinal nematodes in 13 commercial sheep flocks

Summary The prevalence of drug-resistant ovine parasites in the United States has not been widely reported. Thirteen flocks, typical of commercial sheep production units, were selected for survey. Four anthelmintics (fenbendazole, ivermectin, pyrantel pamoate, and levamisole) were tested for their ability to reduce herd mean pretreatment fecal egg count. If a properly dosed and administered drug failed to reduce herd mean pretreatment fecal egg count by 80%, it was considered ineffective in that flock, and the presence of parasites resistant to that drug was inferred. Fenbendazole administration changed pretreatment fecal egg counts by +9% to −100%. On the basis of the aforementioned definition, drug resistance existed in 6 of 13 flocks. Posttreatment larval culture indicated that Haemonchus contortus survived administration of fenbendazole. Levamisole, pyrantel pamoate, and ivermectin reduced pretreatment fecal egg count by −83% to −100%; resistance to these products was not evident in the flocks surveyed.

A field evaluation of benzimidazole and nonbenzimidazole drugs in a herd of dairy goats.

A study was conducted to investigate the efficacy of six anthelmintics in a herd of dairy goats. Pretreatment larval cultures indicated that the goats were infected with Haemonchus contortus and Trichostrongylus colubriformis. Three separate treatment regimens were administered. In each trial, mature nonlactating goats were allocated into two treatment groups and a control group. Treatment groups received thiabendazole (TBZ) or levamisole (LEV), mebendazole (MBZ) or fenbendazole (FBZ), and morantel tartrate (MOR) or ivermectin (IVR). LEV, MOR, and IVR reduced fecal strongyle egg counts by 99% to 100% of pretreatment values. The benzimidazole (BZD) drugs changed pretreatment fecal egg counts by +2% to -32%. Results of posttreatment larval culture demonstrated the presence of H contortus larvae following the administration of BZD drugs.